boehm-gc/include/backptr.h

/*
 * This is a simple API to implement pointer back tracing, i.e.
 * to answer questions such as "who is pointing to this" or
 * "why is this object being retained by the collector"
 *
 * This API assumes that we have an ANSI C compiler.
 *
 * Most of these calls yield useful information on only after
 * a garbage collection.  Usually the client will first force
 * a full collection and then gather information, preferably
 * before much intervening allocation.
 *
 * The implementation of the interface is only about 99.9999%
 * correct.  It is intended to be good enough for profiling,
 * but is not intended to be used with production code.
 *
 * Results are likely to be much more useful if all allocation is
 * accomplished through the debugging allocators.
 *
 * The implementation idea is due to A. Demers.
 */

/* Store information about the object referencing dest in *base_p     */
/* and *offset_p.                                                     */
/* If multiple objects or roots point to dest, the one reported	      */
/* will be the last on used by the garbage collector to trace the     */
/* object.							      */
/*   source is root ==> *base_p = address, *offset_p = 0	      */
/*   source is heap object ==> *base_p != 0, *offset_p = offset       */
/*   Returns 1 on success, 0 if source couldn't be determined.        */
/* Dest can be any address within a heap object.                      */
typedef enum {  GC_UNREFERENCED, /* No reference info available.	*/
		GC_NO_SPACE,	/* Dest not allocated with debug alloc  */
		GC_REFD_FROM_ROOT, /* Referenced directly by root *base_p */
		GC_REFD_FROM_REG,  /* Referenced from a register, i.e.	*/
				   /* a root without an address.	*/
		GC_REFD_FROM_HEAP, /* Referenced from another heap obj. */
		GC_FINALIZER_REFD /* Finalizable and hence accessible.  */
} GC_ref_kind;

GC_ref_kind GC_get_back_ptr_info(void *dest, void **base_p, size_t *offset_p);

/* Generate a random heap address.            */
/* The resulting address is in the heap, but  */
/* not necessarily inside a valid object.     */
void * GC_generate_random_heap_address(void);

/* Generate a random address inside a valid marked heap object. */
void * GC_generate_random_valid_address(void);

/* Force a garbage collection and generate a backtrace from a */
/* random heap address.                                       */
/* This uses the GC logging mechanism (GC_printf) to produce  */
/* output.  It can often be called from a debugger.  The      */
/* source in dbg_mlc.c also serves as a sample client.	      */
void GC_generate_random_backtrace(void);

/* Print a backtrace from a specific address.  Used by the 	*/
/* above.  The client should call GC_gcollect() immediately	*/
/* before invocation.						*/
void GC_print_backtrace(void *);